recently I have migrated Kubuntu and KDE and removed everything related to Gnome (with some exceptions of course). I've read some and learnt that Kubuntu has its own repositories. So, what I wonder is in what manner I have to change my repository list. I mean which part of my list should be discarded and which parts should be kept. Here is my sources.list:

# deb cdrom:[Ubuntu 12.04 LTS _Precise Pangolin_ - Release amd64 (20120425)]/ dists/precise/main/binary-i386/
# deb cdrom:[Ubuntu 12.04 LTS _Precise Pangolin_ - Release amd64 (20120425)]/ dists/precise/restricted/binary-i386/
# deb cdrom:[Ubuntu 12.04 LTS _Precise Pangolin_ - Release amd64 (20120425)]/ precise main restricted
deb-src http://archive.ubuntu.com/ubuntu precise main restricted #Added by software-properties
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://tr.archive.ubuntu.com/ubuntu/ precise main restricted
deb-src http://tr.archive.ubuntu.com/ubuntu/ precise restricted main multiverse universe #Added by software-properties
## Major bug fix updates produced after the final release of the
## distribution.
deb http://tr.archive.ubuntu.com/ubuntu/ precise-updates main restricted
deb-src http://tr.archive.ubuntu.com/ubuntu/ precise-updates restricted main multiverse universe #Added by software-properties
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://tr.archive.ubuntu.com/ubuntu/ precise universe
deb http://tr.archive.ubuntu.com/ubuntu/ precise-updates universe
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://tr.archive.ubuntu.com/ubuntu/ precise multiverse
deb http://tr.archive.ubuntu.com/ubuntu/ precise-updates multiverse
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://tr.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
deb-src http://tr.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse #Added by software-properties
deb http://security.ubuntu.com/ubuntu precise-security main restricted
deb-src http://security.ubuntu.com/ubuntu precise-security restricted main multiverse universe #Added by software-properties
deb http://security.ubuntu.com/ubuntu precise-security universe
deb http://security.ubuntu.com/ubuntu precise-security multiverse
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
deb http://archive.canonical.com/ubuntu precise partner
# deb-src http://archive.canonical.com/ubuntu precise partner
## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.
deb http://extras.ubuntu.com/ubuntu precise main
# deb-src http://extras.ubuntu.com/ubuntu precise main
deb http://tr.archive.ubuntu.com/ubuntu/ precise-proposed restricted main multiverse universe

Yes, as izx said, you shouldn't remove any repositories and Kubuntu has the same core packages available as Ubuntu, but different ones installed.

However, one of the first things I do in a new install of Kubuntu is adding ppa:kubuntu-ppa/ppa, because the official repositories don't push new KDE versions... I recommend it to you as well; I never had problems with it, because new KDE versions get properly tested there too.

ppa:kubuntu-ppa/backports may also be worth adding when a new version of Ubuntu is released and you use an older one and don't want to upgrade. This repository will keep getting new KDE versions while the .../ppa one will get quiet and move on to the next version of Ubuntu.